The present invention relates to electrical connectors and, more particularly, to electrical connectors for interconnecting electrical circuit members such as printed circuit boards, circuit modules, or the like, which may be used in information handling system (computer) or telecommunications environments.
The current trend in the design of high speed electronic systems is to provide both high density and highly reliable interconnections between various circuit devices, which form important parts of these systems. The system may be a computer, a telecommunications network device, a handheld xe2x80x9cpersonal digital assistantxe2x80x9d, medical equipment, or any other electronic equipment. High reliability for such connections is essential due to potential end product failure, should vital misconnections of these devices occur. Further, to assure effective repair, upgrade, and/or replacement of various system components (i.e., connectors, cards, chips, boards, modules, etc.), it is also highly desirable that such connections be separable and reconnectable in the field or at least reworkable at the factory. Also, with financial pressures on manufacturers to be cost competitive, it is also important that high-dollar-value assemblies be reworkable to maximize yields and minimize material to be scrapped.
Today""s ceramic and polymeric composite modules are available in a wide variety of sizes and complexities ranging from small, single-chip modules with a few layers of wiring to multi-chip modules greater than 100 mm square, supporting over 100 chips and containing up to 90 layers of wiring. These modules have a coefficient of thermal expansion (CTE) that is significantly different from the mating structure, typically an FR4-based system board. For field separable applications, these modules predominantly use pin-and-socket or pin-grid-array (PGA) connectors for electrical interconnection to a system board. But for applications that do not require field separability, the most common methods of interconnection are ball grid array (BGA) and column grid array (CGA) technology. The two methods are similar in that they both comprise an array of solder connections, with a primary difference being that the CGA solder connections are taller. For a given module size, the taller CGA solder connections are more tolerant of the CTE mismatch, thus allowing the module and system board to be interconnected reliably with less concern for cracked solder joints.
As both the module size and the distance that the solder connections are located from the center of the module (i.e., the distance from neutral point (DNP)) continue to increase, even CGA solder connections can become unreliable especially due to thermal fatigue. These same concerns about broken interconnections also apply to PGA applications when the DNP is too large. Therefore, what is needed is an interconnection that is relatively flexible and capable of accommodating the thermal mismatch over an expected temperature range that can closely match the electrical performance and density of a BGA or CGA array.
One solution is to use a land grid array (LGA) connector. An array of interconnection elements, known as an interposer, is placed between the two arrays to be connected, and provides the electrical connection between the contact points or pads. Since the individual contact members of an interposer can be made resilient, they can accommodate the CTE mismatch between the module and system board. But since a retentive force is not inherent as in a pin-and-socket type interconnection, a clamping mechanism is needed to create the force necessary to ensure that each contact member is suitably compressed during engagement. This forms the required interconnections to the contact pads on the module and system board. While LGA interposers and clamps are implemented in many different ways, the implementations of most interest are those described in the aforementioned copending U.S. patent applications.
For factory reworkable applications, an LGA connector with its clamping mechanism may be more complex and costly than desired. But a significant advance in the art results when the ends of the contact members of an LGA connector are semi-permanently attached to contact pads on both the module and the system board. This technique provides all of the benefits of a reworkable interconnection, but with contact members providing significantly improved resilience and therefore improved reliability to handle CTE mismatches. Also, for certain field separable applications, the semi-permanent attachment of one end of an LGA connector to either the module or the system board can provide increased reliability, since there is only one field separable interface. Field separability allows the opportunity to upgrade or replace one or more defective devices without the entire system being returned to the factory or discarded.
It is therefore an object of the invention to provide a reworkable LGA-based interconnection for use between two primarily parallel circuit members.
It is an additional object of the invention to provide a reworkable LGA-based interconnection with resilient contact members.
It is another object of the invention to provide a reworkable LGA-based interconnection with enhanced retention.
It is another object of the invention to provide a reworkable LGA-based interconnection with enhanced reliability.
It is another object of the invention to provide a reworkable LGA-based interconnection that is cost effective to manufacture and assemble.
It is a still further object of the invention to provide a reworkable LGA-based interconnection that is field separable at one end.
The present invention is a cost effective, reworkable, LGA-based interconnection for use between two primarily parallel circuit members such as a ceramic module and an FR4-based system board. The resilient contact members allow the reliable interconnection of two circuit members, even though the circuit members may have significantly different CTEs and have interconnections where the DNP is great enough to crack BGA or CGA solder connections. For factory reworkable applications, the ends of the contact members are semi-permanently attached to both the module and the system board. For certain field separable applications, semi-permanently attaching only one of the ends of an LGA to either the module or the system board provides increased reliability due to only one field separable interface. The interconnection carrier provides improved retention of the individual contact members, resulting in interconnections with improved manufacturability, reliability and more uniform mechanical and electrical performance.